Caps are commonly used to close an opening of a device to protect internal components of the device or materials contained in the device. Some caps function to shield the interior components or materials from the atmosphere around them, whereas others serve to protect against the escape of an interior component or material that may harm the environment. In some instances the cap performs both functions. One example is a cap used with an air-conditioning system service valve. The cap is used to both seal the valve internal components from moisture in the environment and to protect the environment from potential leakage of refrigerant from the valve.
Caps have employed various devices to create seals, including metal-to-metal material deformation, hard gaskets, soft gaskets, etc. For soft seals, typically a gland (groove) is provided for the sealing gasket. The gland serves to locate the gasket as well as provide surfaces suitable for creating a seal with the gasket. Unfortunately, when adding a gasket to a gland in a cap, air can be trapped between the cap and the gasket. This condition is made worse when compressing the gasket to complete the seal as the volume available for the trapped air will be reduced. The trapped air, particularly when under pressure, can force the gasket out of the cap gland when the cap is unthreaded from body to which it had been attached.